I am working at the moment on an issue where we are seeing CPU Usage issues on a particular host when converting images using iMagick. The issue is pretty perfectly described here: https://github.com/ResponsiveImagesCG/wp-tevko-responsive-images/issues/150 (I don't use that particular library, but I DO use the same responsive images classes they do, and I am timing out on that particular line, only for some images).
They seem to suggest that removing the call to ->posterizeImage() will fix their issue, and in my tests it does, I can't even tell any difference in the converted images. But this worries me because I wonder if there is a difference that I am not seeing, or one that only comes up in certain scenarios (I mean if posterizing an image didn't do anything there wouldn't be a method for it, right?). I see online that it 'Reduces the image to a limited number of color level' (136 levels in the case causing an issue for me, for what it's worth). I'm having some difficulty parsing that though, which I think is related to a poor grasp of the way various image formats store data (really it doesn't go past the idea that an image is broken up into pixels, which are broken up into proportions of red green and blue).
What actual visual differences could I expect to see if we stop posterizing images? Is it something that I would only expect in certain types of image (like, would it be more visible in transparent over non-transparent, or warmer coloured images)? Or that would be more evident in certain display styles (like print, or the warmer colour temp in iPhone displays)?
Basically I am looking for the info to make an informed choice on whether it's safe to comment out. I'm not worried if it means some images might be x Kb larger, but if it will make them look poor quality, or distort them in some way (even in corner cases) then I need to consider other options.
From the ImageMagick command line documentation:
-posterize levels
reduce the image to a limited number of color levels per channel.
Very low values of levels, e.g., 2, 3, 4, have the most visible effect.
There is a bit more info in the Color Quantization examples - it also has some example images:
The operators original purpose (using an argument of '2') is to re-color images using just 8 basic colors, as if the image was generated using a simple and cheap poster printing method using just the basic colors. Thus the operator gets its name.
...
An argument of '3' will map image colors based on a colormap of 27 colors, including mid-tone colors. While an argument of '4' will generate a 64 color colortable, and '5' generates a 125 color colormap.
Essentially it reduces the number of colors used in the image - and by extension the size. Using a level of 136 would not have much visible affect, as this translates to a 2,515,456 color colortable (136^3).
It is also worth noting from the commit for the issue you linked is that this isn't even always an effective way of reducing image size:
... it turns out that posterization only improves file sizes for PNGs and can actually lead to slightly larger file sizes for JPG images.